(1) Field of the Invention
The invention generally relates to a television transmission system for transmitting television pictures in a digital format from an encoding station to a decoding station via some transmission medium.
More particularly, the invention relates to a television transmission system in which the encoding station comprises a picture transform circuit for performing a forward transform to generate so-called basic picture weighting factors and in which the decoding station comprises a weighting factor transform circuit for performing an inverse transform.
Such a television system may form part of a television broadcasting system, in which case the encoding station forms part of the television broadcasting transmitter and each TV receiver is provided with a decoding station. In this case the transmission medium is the atmosphere.
Such a system may also form part of a video recorder, in which case the video tape is the transmission medium.
(2) Description of the Prior Art
In picture encoding it is common practice to consider a picture as a matrix of E.sub.1 .times.E.sub.2 pixels each having a given luminance and color, if any. It will hereinafter be assumed that the picture only comprises half tones and the luminance of a pixel will be indicated by means of the pixel value. When digitising such a picture, a number (in a binary form) is assigned to each pixel. This number may indicate the pixel value itself, or, for example the difference between the pixel values of two contiguous pixels. In the former case a digital picture in a canonical form, or shortly a canonical picture is concerned.
If 8 bits are used for representing the pixel value of a canonical picture, this means that a conventional canonical picture of 576.times.720 pixels requires approximately 3.times.10.sup.6 bits for its representation, which with a transmission of 25 pictures per second leads to a bit rate of approximately 75.times.10.sup.6 bits/second. Generally, this is unacceptbly high. The object of the encoding station is to convert this canonical picture into a non-canonical picture which can be represented with a considerably lower number of bits.
Different methods are known for this conversion, for example the above-mentioned method in which a number is assigned to each pixel, indicating the difference between the pixel values of two contiguous pixels. This method is known under the name of Differential Pulscode Modulation, abbreviated DPCM. Another method is to subject the canonical picture to a forward picture transform. To this end the picture is partitioned into sub-pictures of N.sub.1 .times.N.sub.2 pixels each, which are each considered as a sum of mutually orthogonal basic pictures B.sub.q,r also of N.sub.1 .times.N.sub.2 pixels each and each with its own weighting factor Y(B.sub.q,r). As a result of the correlation between the pixels of a sub-picture the information is concentrated in a limited number of basic pictures. Only the associated weighting factors are important and the other weighting factors can be ignored.
To determine the weighting factors, N.sub.1 is generally taken to be equal to N.sub.2 in practice and a sub-picture is mathematically considered as an N.times.N matrix X and the weighting factors are also arranged in accordance with an N.times.N matrix which will be denoted by Y. Furthermore, an orthogonal N.times.N transform matrix A is defined which is related to the chosen collection of basic pictures B.sub.q,r. More particularly it holds that: EQU B.sub.q,r =A.sub.q A.sub.r.sup.T ( 1)
Here A.sub.q represents an N.times.N matrix in which each column is equal to the q-th column of the transform matrix A and A.sub.r.sup.T represents a matrix each row of which is equal to the r-th row of the matrix A. The said weighting factors now follow from the matrix multiplication EQU Y=A.sup.T .times.A (2)
In this expression A.sup.T represents the transposed matrix of A.
For more information on the above, reference is made to Reference 1.
The number of weighting factors which must be transmitted appears to be closely related to the structure of the chosen basic pictures and, in conjunction therewith the chosen transform-matrix A. In this connection the Karhunen-Loeve transform matrix (see for example Reference 2, pages 259-264) is found to be optimum. However, it is difficult to implement. Nowadays the discrete cosine transform, abbreviated DCT is generally considered to be the best alternative (see Reference 2).
In practice it is found that the number of weighting factors qualified for transmission may greatly differ from sub-picture to sub-picture. To be able to make a selection, all coefficients must always be calculated.